Halogenated bis-imide flame retardants

ABSTRACT

This invention relates to a process for enhancing the properties of alkylene(C 1  -C 6 )-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) products produced by the bromination, respectively, of alkylene(C 1  -C 6 )-bis-(phthalimide) or bis-(phthalimide) with a brominating agent in the presence of oleum, the process comprising curing the alkylene(C 1  -C 6 )bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) products at a temperature above about 150° C. and below the melting point of the tetrabromophthalimide being cured for a period of from about 36 hours to about 5 hours. This enhanced product and its use in molding and extrusion compositions and in molded and extruded articles is also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to improved halogenated bis-imide flame retardants, to their manufacture and to compositions containing a flammable material and such halogenated bis-imides.

As is taught in U.S. 4,374,220, there are a multitude of halogenated bis-imides which are effective as flame retardants in composition with macromolecular flammable materials, e.g., polymers. These compositions are useful in making articles such as wire insulation coverings and electronic housings. Of these halogenated bis-imides, the alkylene(C₁ -C₆)-(tetrabromophthalimide)s are especially commercially significant.

A presently used commercial route for producing alkylene (C₁ -C₆)-bis-(tetrabromophthalimide) products comprises reacting tetrabromophthalic anhydride with a diaminoalkane in the presence of water and an alkanoic acid to yield an alkylene-diammonium-bis-(tetrabromophthalate). The reaction mass is then heated to about 210° C. for a period of about 8 hours and the resultant desired alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product is recovered therefrom. This product is particularly useful as it has good thermal stability and resistance to UV degradation. However, the product has a yellow color which argues against its presence in compositions used for forming white articles. Also, the intensity of the yellow color can vary between product batches, which color variance makes it difficult for the article manufacturer to maintain consistency in the color of the articles produced. To overcome the yellow color problem, the art has provided processes which produce a white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product.

U.S. Pat. No. 4,125,535 discloses a process for producing a white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product by reacting tetrabromophthalic anhydride with diaminoalkane in a solvent system comprised of a liquid aromatic hydrocarbon and an alkanoic or aralkanoic acid.

Another process for producing a white bis-imide product is disclosed in European Patent No. 0 023 420. This process comprises the bromination of alkylene(C₁ -C₆)-bis-(phthalimide) with a brominating agent, e.g. bromine, in 40-80% oleum. The product is white, however, it is not as thermally stable or as resistent to UV degradation as is the case for the before-described yellow alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product. Also, it has now been demonstrated, that when compositions containing this white bis-imide product are compression molded to form articles, the resultant article can be discolored by a yellow color or tint. This discoloration is believed to be due to a yellowing of this bis-imide product as a result of its being exposed to the compression molding process conditions. Since injection molding processes are usually run under process conditions which are more harsh than those used in compression molding, it is expected, for this bis-imide, the the yellowing problem which would be encountered in injection molding would be at least as severe as that encountered in compression molding.

It is, therefore, an object of this invention to provide a white bis-imide, i.e. alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product, which has high thermal stability, resistance to UV degradation and which does not significantly contribute a yellow color to an article made from a composition containing such bis-imide product.

It is also an object of this invention to provide a process for producing such a white bis-imide product.

It is also an object of this invention to provide a composition which contains a macromolecular flammable constituent and a white bis-imide product of the invention wherein the color of the composition is not significantly affected by a yellow color contribution from the bis-imide product during the processing of the composition to form an article.

These and other objects are attained by oven curing a alkylene (C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product which has been produced, respectively, by the bromination of alkylene(C₁ -C₆)-bis-(phthalimide) or bis-(phthalimide) with a brominating agent in the presence of oleum.

THE INVENTION

For the purpose of this disclosure, the alkylene(C₁ -C₆)-bis-(phthalimide)s and the bis-(phthalimide) referred to herein are represented by the formula, ##STR1## wherein R is a divalent alkylene radical containing 1 to 6 carbon atoms and a is 1 or 0. R can be branched or be a straight chain radical. R is preferably methylene(--CHhd 2--) or ethylene(--CH₂ CH₂ --). When a is 0, the bonding between the two phthalimide groups is by way of a N--N bond.

The alkylene(C₁ -C₆)-bis-(tetrabromophthalimide)s and the bis-(tetrabromophthalimide)s referred to herein are represented by the formula, ##STR2## wherein R and a are as recited above.

The oleum route, as before noted, to a white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product comprises the bromination of alkylene(C₁ -C₆)-bis-(phthalimide) with a brominating agent in the presence of oleum.

The brominating agent used is usually bromine, although other brominating agents, such as bromine chloride, can be employed if desired. The amount of bromine used should be at least the amount theoretically required to achieve complete bromination of the aromatic rings of the starting material, i.e., an amount that provides one bromine atom for each available aromatic carbon atom. There is no maximum to the amount of bromine that can be used, but there is no advantage in using unnecessarily large amounts of bromine. Therefore, it is preferable to employ amounts of bromine that provide from about 1 to about 1.5, most preferably from about 1.05 to about 1.15, atoms of bromine per available aromatic carbon atom. Excellent results are obtained by using bromine in an amount that is about 12-14% more than the amount theoretically required for complete bromination.

The oleum used is a solution of 40-80% by weight of sulfur trioxide in concentrated sulfuric acid. Because of its ready availability and its effectiveness, a 60-70% oleum is preferred. The oleum is employed in an amount which provides at least one molecule of sulfur trioxide per available aromatic carbon atom and is preferably used in larger amounts since the reaction rate increases with an increased concentration of sulfur trioxide in the reaction mixture. Ordinarily the oleum is utilized in an amount which provides from about 1 to about 1.9, preferably from about 1.25 to about 1.75, molecules of sulfur trioxide per available aromatic carbon atom. Thus, it is preferable to use from about 8 to about 15 mols, most preferably from about 10 to about 14 mols, of suflur trioxide per mol of alkylene(C₁ -C₆)-bis-(phthalimide). Excellent results are obtained by using about 50% more sulfur trioxide than is theoretically required for complete sulfonation of the aromatic rings of the bis-imide reactant. When an oleum containing the desired amount of sulfur trioxide is not readily available, the desired concentration of sulfur trioxide can be attained by introducing the sulfur trioxide to a sulfur trioxide deficient system.

In the reaction, relatively mild conditions are suitable. It is not necessary to use high temperatures or pressures. Ordinarily the desired bromination can be achieved by heating the reactants to a temperature of from about 35 to about 60° C., preferably about 55° C., for at least about 3 hours, although it is sometimes desirable to use higher temperatures, e.g. about 70-90° C., to complete the bromination when the concentration of sulfur trioxide oxide in the reaction mixture is relatively low during the initial stage of the reaction. Such relatively low concentrations in the reaction mixture may occur, for example, if a whole charge of alkylene(C₁ -C₆)-bis-(phthalimide) is initially present, as distinguished from being added gradually to the oleum.

The reaction may be conducted under atmospheric or superatmospheric pressure, with or without a catalyst. When pressure is employed, it is ordinarily only the slight pressure created by a back pressure device, although higher pressures may be used if desired. When a catalyst is employed, it may be any suitable bromination catalyst, e.g., sodium nitrate, nitrogen dioxide, aluminum, iron, an iron-iodine mixture, a silver catalyst, etc. Catalysts, when employed, are normally used in concentrations of about 0.1-2%, based on the weight of the alkylene(C₁ -C₆)-bis-(phthalimide). Particlarly good results are obtained when the catalyst is a 50:50 mixture of iron and iodine, and even better results are obtained when the catalyst is a silver catalyst, e.g., silver or any inorganic or organometallic silver compound capable of catalyzing the bromination of aromatic carbons. Because of availability as well as effectiveness, the preferred silver catalysts are silver, silver nitrate, silver sulfate, silver carbonate, silver acetate, silver chloride, silver bromide, and mixtures thereof. Silver nitrate is particularly preferred.

The manner of admixing the reactants is not critical, although the reaction is much more efficient when the reactants are admixed in such a manner to maximize the concentration of sulfur trioxide at all times. One suitable method of conducting the reaction is to dissolve the alkylene(C₁ -C₆)-bis-(phthalimide) and optional catalyst in the oleum, to heat the solution to about 35-60° C., to add the brominating agent thereto over a period of about 3-10 hours, to maintain the reaction mixture at about 35-60° C. for an additional 0-5 hours, to raise the reaction temperature to about 70-90° C., and to maintain the reaction mixture at about 70-90° C. for about 1-3 hours.

A preferred method of conducting the reaction is to charge optional catalyst and about 45-75% of the oleum to a reaction vessel, to heat the oleum and optional catalyst to about 35-60° C., and to simultaneously add the bromine and a solution of the alkylene(C₁ -C₆)-bis-(phthalimide) in the remainder of the oleum over a period of about 3-5 hours.

When the bromination has been completed, the alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product is recovered by conventional techniques. Conveniently the product is isolated by raising the temperature of the reaction mass to about 100-140° C. to distill off the excess bromine and sulfur trioxide. The reaction mass is then cooled to room temperature and filtered. The recovered precipitate, i.e. an alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product, is washed with water until it is free of acid and then dried. The drying is generally accomplished by heating the washed precipitate to about 125° C. for several hours.

The production of a white bis-(tetrabromophthalimide) product is effected in a manner similar to that recited above for the alkylene (C₁ -C₆)-bis-(tetrabromophthalimide) product except that bis-(phthalimide) intermediate is used in place of the alkylene(C₁ -C₆)-bis-(phthalimide) intermediate.

The so formed and recovered white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product has, as before stated, a thermal stability and resistance to UV degradation which is inferior to that of the yellow alkylene(C₁ -C₆)-bis-tetrabromophthalimide) product produced by the reaction of tetrabromophthalic anhydride and diaminoalkane. To enhance the white products of this invention so that they are substantially equivalent to the yellow product, in regards to thermal stability and resistance to UV degradation, it has now been found that a further process step is useful. In accordance with this invention, this enhancement is accomplished by curing the white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product at a temperature above about 150° C. and below the melting point of the tetrabromophthalimide, i.e. the alkylene (C₁ -C₆)-bis-tetrabromophthalimide or the bis-tetrabromophthalimide being cured, for a period within the range of from about 36 hours to about 5 hours. Generally, the white bis-imide product, of this invention can be cured at a temperature of from about 150° C. to 300° C. without melting. The longer curing periods are associated with the lower curing temperatures and the shorter curing periods are associated with the higher curing temperatures. Preferably, the curing temperature is within the range of from about 180° C. to about 250° C. with a correspondent curing period within the range of from about 10 hours to about 6 hours. A most highly preferred curing temperature will be within the range of from about 220° C. to about 250° C. in association with a curing period within the range of from about 8 to about 6 hours. The curing period can be longer than those specified above for a certain temperature, and such periods have been used. However, no particular benefit is gained in using excessively long curing periods. The curing can be effected with any conventional technique and apparatus, provided that, the technique and apparatus chosen assures uniform curing and does not deleteriously degrade or contaminate the product being cured. The curing can occur at subatmospheric, atmospheric or at superatmospheric pressures dependent upon the technique and apparatus used. An especially useful technique is oven curing in which a hot forced air oven is used. The product is best cured by being placed in the oven in thin layers, e.g. 0.25 in. to 4 in., to insure uniform curing. The specification of layer thickness is not critical so long as uniform curing is obtained.

The cured white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) products (hereinafter referred to as cured white bis-imide product) not only has good thermal stability and resistance to UV degradation but also has a lower acid number, less than about 2, and a higher bromine content, within the range of from about 63% to about 69%, than is the case for the precursor uncured white alkylene(C₁ -C₆)-bis-(tetrabromophthalimide and bis-(tetrabromophthalimide) products (hereinafter referred to as uncured white bis-imide product). Indeed, for cured ethylene-bis-(tetrabromophthalimide) an acid number within the range of from about 0.15 to about 1.0 and a bromine content within the range of from about 65% to about 67% are obtainable. A preferred acid number range for this cured white bis-imide product is from about 0.15 to about 0.9.

The cured white bis-imide products of this inventive function as flame retardants when compounded with macromolecular flammable materials. The compositions so formed can additionally contain other conventional additives such as stabilizers, processing aids and the like.

It will be apparent to those skilled in the art that for all cases no single precise value for the proportion of the cured white bis-imide product in the composition can be given, since this proportion will vary with the particular flammable material, the presence of other additives, the particular cured white bis-imide product used, and the degree of flame retarding sought in any given application. Further, the proportion necessary to achieve a given extent of flame retardance in a particular composition will depend upon the shape of the article into which the composition is to be made, for example, electrical insulation, tubing and film will each behave differently. In general, however, the composition may contain from about 5% to about 40%, preferably 10% to 30%, of the cured white bis-imide product when the cured white bis-imide product is the only flame retardant compound in the composition.

It will be appreciated that one or more bis-imides, be they the cured white bis-imide products of this invention or be they a mix of cured and uncured white bis-imide products, may be incorporated in the same composition and that other flame retardant materials may be used in conjunction with the bis-imides. (Note, however, that the use of uncured white bis-imide products may create color and degradation problems if present in substantial amounts.) It is especially advantageous to use the cured white bis-imide product and an inorganic compound, especially the oxide, of a Group V B element, for example, bismuth, arsenic, phosphorus and especially antimony, in a composition. Of these compounds, antimony oxide is especially preferred. If such a compound is present in a composition, the quantity of total bis-imide product, be it all cured or a mix of cured and uncured, needed to achieve a given flame-retardance is accordingly reduced.

The compositions containing the total bis-imide product, inorganic compound flame retardant system may contain up to about 40% by weight of the system, preferably between 20% and 30% by weight. It is believed that the inorganic compound and the total bis-imide product will react under the conditions of combustion of a flammable material to form inorganic bromine compounds, e.g., bromine and oxybromides, which assist in retarding combustion. The bromine-bearing bis-imide also acts as a flame retardant independently and the proportions in a flame retardant system are a matter of choice, depending, inter alia, on the material in which the system is to be incorporated and commercial considerations. Generally, the total bis-imide product and the inorganic compound are in a weight ratio of from about 1:1 to about 7:1, and preferably of from about 2:1 to about 4:1.

The cured white bis-imide products and flame retardant systems containing cured white bis-imide products may be used in combination with virtually any flammable material. The material may be macromolecular, for example, a cellulosic material or a polymer. Illustrative polymers are: olefin polymers, cross-linked and otherwise, for example, homopolymers of ethylene, propylene, and butylene; copolymers of two or more of such alkylene monomers and copolymers of one or more of such alkylene monomers and any other copolymerizable monomers, for example, ethylene/propylene copolymers, ethylene/ethyl acrylate copolymers and ethylene/vinyl acetate copolymers; polymers of olefinically unsaturated monomers, for example, polystyrene, e.g. high impact polystyrene, and styrene copolymers; polyurethanes; polyamides; polyimides; polycarbonates; polyethers; acrylic resins; polyesters, especially poly(methyleneterephthalate), poly(ethyleneterephthalate) and poly(butyleneterephthalate); epoxy resins; alkyds; phenolics; elastomers, for example, butadiene/styrene copolymers and butadiene/acrylonitrile copolymers; terpolymers of acrylonitrile, butadiene and styrene; natural rubber; butyl rubber; and polysiloxanes. The polymer may also be a blend of various polymers. The composition may contain any of the additives usually present and where appropriate to the particular polymers, may be crosslinked by chemical means or by irradiation.

It is a feature of the cured white bis-imide products of this invention that their presence in a composition will not substantially contribute a yellow color or tint to an article formed from the composition by conventional molding techniques. For example, compositions containing white high impact polystyrene (HIPS) and from about 9 wt. % to about 15 wt. % cured white ehtylene-bis-(tetrabromophthalimide) product, the latter having a Yellowness Index of about 10, can be used to produce molded articles having a Yellowness Index within the range of from about 16 to about 23. Also, in compositions containing from about 8 wt. % to about 14 wt. % of the same ethylene bis-imide product and white poly(butyleneterephthalate), a Yellowness Index within the range of from about 8 to about 13 is attainable.

The inventions disclosed herein are illustrated in the following Example.

The ethylene-bis-(phthalimide) reactant used in the Example was prepared in accordance with the following procedure.

A 5 liter resin kettle equipped with a stirrer, thermometer, condenser, Barret trap and heating mantle was charged with 1378 g of xylene and 590.5 g of acetic acid. The mixture was heated to 60° C. and stirred. Ethylene diamine (202.8 g) was dissolved in 200 g (234 cc) of xylene and kept separate. When the reaction temperature reached 60° C., phthalic anhydride was added to the kettle in ten increments of 100 g each, at ten minute intervals. The ethylene diamine/xylene solution was simultaneously added in ten increments of 42.55 cc each, at ten minute intervals. When the additions were complete, the reaction temperature was brought to 105° C. and then maintained within the range of 105° C. to 125° C. for a period of three hours. The reaction mass was cooled to room temperature and filtered. The filter cake was washed twice with xylene (500 cc each time) and 16 times with water (500 cc each time). The wet cake was dried at a temperature of 125° C. in a forced hot air oven overnight to give 1013 g (94% yield) of ethylene-bis-(phthalimide) which was off-white in color and had a melting point of 239°-241° C. and an acid value of 0.11.

EXAMPLE I

For comparative purposes, yellow ethylene-bis-(tetrabromophthalimide), i.e. Ethyl Corporation's BT-93™ flame retardant, was obtained and uncured white ethylene-bis-(tetrabromophthalimide) and cured white ethylene-bis-tetrabromophthalimide) were prepared.

Preparation of Uncured White Ethylene-bis-(tetrabromophthalimide)

A 3 liter resin kettle equipped with a stirrer, thermometer, thermowatch, double glycol condenser, addition funnel and heating mantle was charged with 1464 g of 65% oleum and 240.2 g of ethylene-bis-(phthalimide). The contents were stirred and heated to 55° C. Bromine (583 g) was then added dropwise over a period of five hours. After the addition was complete, the reaction mass was heated to reflux (60°-63° C.) and maintained at that temperature for two hours. The reaction mass was further heated to 85° C. and stirred at that temperature for another eight hours. The unreacted bromine and SO₃ were distilled off. The reaction slurry was cooled to 30° C. and poured slowly over a large amount of ice. The product was washed with water eight times (3 liter of water each time) and then filtered. The filtered cake was dried at 125° C. overnight to give 683 g (96% yield) of the white ethylene-bis-(tetrabromophthalimide) product with a melting point of 456°-458° C.

Preparation of Cured White Ethylene-bis-(tetrabromophthalimide)

White ethylene-bis-(tetrabromophthalimide) product was prepared as above. Additionally, the following process step was performed. 643 g of previously dried (125° C.) white ethylene-bis-(tetrabromophthalimide) product was placed as a thin layer, about 1/2 inch thick, on a sheet in a forced hot air oven. The product was then cured at a temperature of about 250° C. for 18 hours to yield 623 g of the material.

Thermogravimetric analyses were performed on the above produced yellow, uncured white and cured white ethylene-bis-(tetrabromophthalimide) products with a duPont 1090 Thermal Analyzer. Table I gives the results.

In the tables, EBT is defined as ethylene-bis-(tetrabromophthalimide) product.

                  TABLE I                                                          ______________________________________                                         TGA Results                                                                               Weight % Remaining                                                             at Indicated Temperature                                                       220° C.                                                                       250° C.                                                                         350° C.                                                                         400° C.                                                                       440° C.                          ______________________________________                                         Uncured white EBT                                                                           99      98      93    90    82                                    Cured white EBT                                                                             100     100     99    97    90                                    Yellow EBT   100     100     99    98    92                                    ______________________________________                                    

Table II gives a % Br and acid number comparison between the uncured white and cured white ethylene-bis-(tetrabromophthalimide) products.

                  TABLE II                                                         ______________________________________                                                         % Br  Acid No.                                                 ______________________________________                                         Uncured white EBT 65.5    4.21                                                 Cured white EBT   67.0    0.17                                                 ______________________________________                                    

Uncured white and cured white ethylene-bis-(tetrabromophthalimide) products, the former having a Yellowness Index of about 5 to about 7 and the latter having a Yellowness Index of about 10 to about 14, were both compounded with white poly(butyleneterephthalate), i.e. VALOX 420, a product of the General Electric Company. The Yellowness Index was determined by a Hunter's Colorometer. The resultant composition was formulated as follows.

    ______________________________________                                                Component                                                                               wt %                                                           ______________________________________                                                VALOX 420                                                                               85                                                                    EBT      10                                                                    Sb.sub.2 O.sub.3                                                                        5                                                              ______________________________________                                    

The compounding was effected by use of a Brabender Mixer at a temperature of about 270° C.

The resultant composition was formed into plaques by compression molding. The compressing molding press was operated at pressure of about 250 psi and at a temperature of about 230° C. with a molding time of 4 minutes. After the plaque was formed in the mold it was cooled to room temperature and then removed from the mold.

The plaques produced from the composition containing the uncured ethylene-bis-(tetrabromophthalimide) product were noticeably yellower than the plaques produced from the composition containing the cured ethylene-bis-(tetrabromophthalimide) product, the latter being essentially yellow color free.

Table III compares the change in color of the plaques upon being exposed to ultraviolet light for 48 hours in a Sunlighter Model 150 manufactured by Test-Lab Apparatus Company of Milford, N.H.

                  TABLE III                                                        ______________________________________                                                   Y.I. (Initial)                                                                             Y.I. (Final)                                                                             ΔE.sub.48                                ______________________________________                                         Plaque containing                                                                          12.75         15.13     3.3                                        uncured white EBT                                                              and VALOX 420                                                                  Plaque containing                                                                          11.03         13.13     2.0                                        cured white EBT                                                                and VALOX 420                                                                  ______________________________________                                    

Uncured white and cured white ethylene-bis-(tetrabromophthalimide) products were compounded with white high impact polystyrene, HIPS 840D, a product of Huntsman Chemical Corp., to form a composition as follows:

    ______________________________________                                                Component                                                                               wt %                                                           ______________________________________                                                HIPS 840D                                                                               84                                                                    EBT      12                                                                    Sb.sub.2 O.sub.3                                                                        4                                                              ______________________________________                                    

The compounding was accomplished with a Brabender Mixer at a temperature of about 175° C.

The composition was formed into plaques by compression molding in a compression molding press using a molding pressure of about 250 psi and at a temperature of about 180° C. with a molding time of about 3 minutes. After the plaque was compression molded, it was cooled to room temperature and then removed from the press.

The plaques containing uncured white ethylene-bis-(tetrabromophthalimide) product were easily seen to have a yellow tint, while, in distinction, the plaques containing cured ethylene-bis-(tetrabromophthalimide) product were not so colored.

Table IV compares the plaques after exposure to ultraviolet light for 48 hours in the same apparatus used to obtain the values reported in Table III.

                  TABLE IV                                                         ______________________________________                                                   Y.I. (Initial)                                                                             Y.I. (final)                                                                             ΔE.sub.48                                ______________________________________                                         Plaques containing                                                                         19.5          43.5      14.8                                       uncured white EBT                                                              and HIPS 840D                                                                  Plaques containing                                                                         16.7          22.6      4.6                                        cured white EBT                                                                and HIPS 840D                                                                  ______________________________________                                     

What is claimed is:
 1. A process for enhancing the properties of a dried alkylene (C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product produced by the bromination, respectively, of alkylene(C₁ -C₆)-bis-(phthalimide) or bis-(phthalimide) with a brominating agent in the presence of oleum, said process comprising curing the alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product at a temperature above about 150° C. and below the melting point of the tetrabromophthalimide being cured for a period within the range of from about 36 hours to about 5 hours.
 2. The process of claim 1 wherein said curing occurs in a hot forced air oven.
 3. The process of claim 1 wherein said temperature is within the range of from about 180° C. to about 250° C. and said period is within the range of from about 10 hours to about 6 hours.
 4. The process of claim 1 wherein said alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product is ethylene-bis-(tetrabromophthalimide).
 5. The process of claim 1 wherein said alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product is methylene-bis-(tetrabromophthalimide).
 6. In a process for preparing an alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product or bis-(tetrabromophthalimide) which process comprises brominating, respectively, alkylene(C₁ -C₆)-bis-(phthalimide) or bis-(phthalimide) with a brominating agent in the presence of oleum, and recovering and drying the produced alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product, the improvement which comprises the additional step of curing the dried alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) or bis-(tetrabromophthalimide) product at a temperature above about 150° C. and below the melting point of the tetrabromophthalimide being cured for a period within the range of from about 36 hours to about 5 hours.
 7. The process of claim 6 wherein said curing occurs in a hot forced air oven.
 8. The process of claim 6 wherein said temperature is within the range of from about 180° C. to about 250° C. and said period is within the range of from about 10 hours to about 6 hours.
 9. The process of claim 6 wherein said alkylene(C_(1-C) ₆)-bis-(tetrabromophthalimide) product is ethylene-bis-(tetrabromophthalimide).
 10. The process of claim 6 wherein said alkylene(C₁ -C₆)-bis-(tetrabromophthalimide) product is methylene-bis-(tetrabromophthalimide). 